In classical physics, free
space is a concept of electromagnetic
theory, corresponding to a theoretically perfect vacuum and sometimes referred to
as the vacuum of free space, or as
classical vacuum, and is appropriately viewed as a
reference medium.[1][2]

The definitions of the ampere and meterSI units are based upon measurements
corrected to refer to free space.[3]

In addition, in free space the principle of linear superposition of potentials and
fields holds: for example, the electric potential generated by two
charges is the simple addition of the potentials generated by each
charge in isolation.[6][7][8]

What is
the vacuum?

Physicists use the term "vacuum" in several ways. One use is to
discuss ideal test results that would occur in a perfect
vacuum, which physicists simply call classical
vacuum[9][10]
or free space in this context. The term
partial vacuum is used to refer to the imperfect
vacua realizable in practice.

The physicist's term "partial vacuum" does suggest one major
source of departure of a realizable vacuum from free space, namely
non-zero pressure. Today, however, the classical concept of vacuum
as a simple void[11]
is replaced by the quantum vacuum, separating "free space" still
further from the real vacuum – quantum vacuum or the vacuum state is not
empty.[12]
An approximate meaning is as follows:[13]

The quantum vacuum is "by no means a simple empty space,"[14]
and again: "it is a mistake to think of any physical vacuum as some
absolutely empty void."[15]
According to quantum mechanics, empty space (the "vacuum") is not
truly empty but instead contains fleeting electromagnetic waves and
particles that pop into and out of existence.[16] One
measurable result of these ephemeral occurrences is the Casimir
effect.[17][18] Other
examples are spontaneous emission[19][20][21]
and the Lamb
shift.[22]
Related to these differences, quantum vacuum differs from free
space in exhibiting nonlinearity in the presence of strong electric
or magnetic fields (violation of linear superposition). Even in
classical physics it was realized [23][24]
that the vacuum must have a field-dependent permittivity in the
strong fields found near point charges. These field-dependent
properties of the quantum vacuum continue to be an active area of
research.[25] The
determined reader can explore various nuances of the quantum vacuum
in Saunders.[26]
A more recent treatment is Genz. [27]

At present, even the meaning of the quantum vacuum state is not
settled. To quote GE Brown:[28]:

“

In eighteen-century
Newtonian mechanics, the three-body problem was insoluble.
With the birth of general relativity around 1910 and quantum
electrodynamics in 1930, the two- and one-body problems became
insoluble. And within modern quantum field theory, the problem of
zero bodies (vacuum) is insoluble. … GE Brown quoted by RD
Mattuck

The discrepancies between free space and the quantum vacuum are
predicted to be very small, and to date there is no suggestion that
these uncertainties affect the use of SI units, whose
implementation is predicated upon the undisputed predictions of quantum electrodynamics.[32]

In short, realization of the ideal of "free space" is not just a
matter of achieving low pressure, as the term partial
vacuum suggests. In fact, "free space" is an abstraction from
nature, a baseline or reference state, that is unattainable in
practice.[33]

Realization of free
space in a laboratory

By "realization" is meant the reduction to practice, or
experimental embodiment, of the term "free space", for example, a
partial vacuum. What is the operational definition of free
space? Although in principle free space is unattainable,
like the absolute
zero of temperature, the SI units are referred to free
space, and so an estimate of the necessary correction to a
real measurement is needed. An example might be a correction for
non-zero pressure of a partial vacuum. Regarding measurements taken
in a real environment (for example, partial vacuum) that are to be
related to "free space", the CIPM cautions that:[3]

“in all cases any necessary corrections be applied to take
account of actual conditions such as diffraction, gravitation or
imperfection in the vacuum.”

In practice, a partial vacuum can be produced in the laboratory
that is a very good realization of free space. Some of the issues
involved in obtaining a high vacuum are described in the article on
ultra high
vacuum. The lowest measurable pressure today is about
10−11 Pa.[34]
(The abbreviation Pa stands for the unit pascal, 1 pascal = 1
N/m2.)

Realization of free
space in outer space

While only a partial vacuum, outer space contains such sparse matter
that the pressure of interstellar space is on the order of
10 pPa
(1×10−11 Pa)[35]. For
comparison, the pressure at sea level (as defined in the unit of atmospheric
pressure) is about 101 kPa (1×105 Pa). The
gases in outer space are not uniformly distributed, of course. The
density of hydrogen in our galaxy is estimated at 1 hydrogen
atom/cm3.[36]
The critical density separating a Universe that continuously
expands from one that ultimately crunches is estimated as about
three hydrogen atoms per thousand liters of space.[37]
In the partial vacuum of outer space, there are small quantities of matter (mostly hydrogen), cosmic dust and cosmic noise. See intergalactic space. In addition, there is
a cosmic
microwave background with a temperature of 2.725 K, which
implies a photon density of about 400 /cm3.[38][39]

The density of the interplanetary medium and interstellar medium, though, is
extremely low; for many applications negligible error is introduced
by treating the interplanetary and interstellar regions as "free
space".

The United
States Patent Office defines free space in a number of
ways. For radio and radar applications the definition is "space
where the movement of energy in any direction is substantially
unimpeded, such as the atmosphere, the ocean, or the earth"
(Glossary in US Patent Class 342, Class Notes).[40]

Another US Patent Office interpretation is Subclass 310:
Communication over free space, where the definition is "a
medium which is not a wire or a waveguide".[41]

^ abCIPM adopted Recommendation 1
(CI-1983) Appendix 1, p. 77 “provided that the given
specifications and accepted good practice are followed; • that in
all cases any necessary corrections be applied to take account of
actual conditions such as diffraction, gravitation or imperfection
in the vacuum; … ”

^Paulo N Correa & Alexandra
N Correa The Sagnac and Michelson-Gale-Pearson Experiments: The
tribulations of general relativity with respect to rotation;
"An absolute vacuum of matter and energy is unattainable and
not a real possibility that should or need be considered. The
"vacuum state" is a misnomer … "